Typical parallel hybrid electric vehicles include an internal combustion engine, an electrical machine, and a battery. Parallel hybrid electric vehicles typically operate using the electric machine and/or the internal combustion engine as prime movers to propel the vehicle. That is, torque may be provided to the vehicle wheels using only the electrical machine, only the engine, or using some combination of torque from both prime movers. Generally, the electrical machine will work as a motor or generator during the period of vehicle acceleration or deceleration, respectively. An inverter may be operably positioned between the electrical machine and the battery to implement the electrical DC or AC power conversion between the electrical machine and the battery.
In a hybrid mode, both the electrical machine and the engine may provide torque to the driven wheels of the vehicle for propulsion. During acceleration or electric only operation, the electrical machine will function as a motor to help propel the vehicle, and the battery will discharge to provide electrical power to drive the motor. During deceleration operation, the electrical machine will function as a generator to charge the battery by absorbing the deceleration mechanical energy.
Because of frequent stop-and-go or acceleration-and-deceleration vehicle operation, the battery will be charged and discharged frequently in a cycling operation. Such cycling operation typically results in heat generation by the battery. The longer the discharging/charging period or the greater the charging/discharging current, the more heat the battery will generate, thus resulting in higher battery temperatures which may impact battery life significantly.
A de-rating of the battery may be used where the battery temperature is monitored and the power supplied by the battery is limited when the battery temperature exceeds a preselected level (or the battery power may be lowered proportionally as the temperature increases). However, this de-rating may not prevent the battery from exceeding an undesirable temperature, and may result in operating the battery unnecessarily at a de-rated capacity. Further, to increase the fuel efficiency of a hybrid vehicle, the battery should operate at the highest capacity possible, and to improve the hybrid battery life, the battery should operate below the desirable temperature.